THREE NATURAL LABORATORIES

Why three natural laboratories? 

No single Archaean terrane preserves the complete history of Earth's earliest continental crust. Each geological region has experienced a unique tectonic, metamorphic and erosional history, preserving different parts of the geological record.

The BICO project combines evidence from three natural laboratories in northern Fennoscandia. Together, they provide an exceptional opportunity to investigate how partial melting of basalt generated the TTG magmas that formed Earth's earliest stable continental crust. By integrating observations from these contrasting geological settings, BICO tests crustal evolution models with greater confidence than would be possible from any single terrane.

Lake Inari Terrain

The Lake Inari Terrain hosts one of the world's most extensive accessible Archaean TTG–metabasalt associations. During the Lapland–Kola Orogeny, the terrane was emplaced as an out-of-sequence thrust sheet onto the Karelian Craton. Much later, block faulting associated with uplift of northern Scandinavia formed the present-day Lake Inari basin, exposing the Archaean terrane while preserving its primary geological character. 

The extensive archipelago provides continuous shoreline exposures over hundreds of kilometres. These exceptional field conditions allow geological relationships to be traced across an entire Archaean terrane rather than between isolated outcrops. Combined with the FIRE deep seismic reflection profile, Lake Inari provides a unique opportunity to investigate the architecture and crust-forming processes of an Archaean TTG–metabasalt terrane. 

Further reading

Halla, J., Joshi, K.B.,  Luttinen, A., Heilimo, E.,  Kurhila, M., 2024. On the origin of Archaean TTGs by migmatization of mantle plume-related metabasalts: Insights from the Lake Inari terrain, Arctic Fennoscandia. Precambrian Research 407, 107407. Open access https://doi.org/10.1016/j.precamres.2024.107407

Joshi, K.B., Halla, J., Kurhila, M., Heilimo, E., 2024. Prolonged parallel chronology of distinct TTG types in the Lake Inari terrain, Arctic Fennoscandia: Implications for a stationary plume-related source. Precambrian Research 408, 107418. https://doi.org/10.1016/j.precamres.2024.107418

Rommaeno Complex

The Rommaeno Complex of northwestern Finnish Lapland exposes some of the oldest crust in northern Europe, with rocks dating back to approximately 3.2 billion years. Across much of the complex, erosion has exposed the Archaean basement at the level of the Great Unconformity, demonstrated by the Caledonian nappes resting directly upon it. 

This rare geological setting provides direct access to the uppermost preserved Archaean continental crust. It reveals the ancient surface that existed before the Cambrian transgression and provides unique insights into how Earth's earliest continents survived billions of years of geological evolution and became stabilised.

Further Reading

West Troms Complex

The West Troms Complex of northern Norway offers extensive exposures of Archaean TTGs and metabasalts along the North Atlantic coast. Long-term uplift, repeated glacial erosion and continuous wave action have created outstanding three-dimensional exposures of the Archaean basement.

These exceptional coastal outcrops allow geological relationships to be traced over long distances and across large areas, providing an outstanding natural laboratory for studying field relationships between Archaean TTGs and metabasalts. Together with structural geology, petrology, geochemistry and geochronology, these observations help reconstruct the crust-forming processes through which partial melting of basalt generated the TTG magmas that built Earth's earliest continental crust.